This invention relates generally to heating polymer matrix composites and more particularly to induction heating of polymer matrix composite fiber strands having electrically conductive fibers for the purpose of partially curing the composite fiber strands.
A composite material combines two or more other materials into a single, integrated structure in a manner that the combined materials maintain their original identities. Polymer matrix composites comprise high-strength fibers embedded in a polymeric matrix. The matrix holds the fibers in the proper orientation and protects them from external damage. Polymeric matrix materials fall into two general classes, thermosetting and thermoplastic. The fibers can be configured in many different formats, depending on the intended application of the composite material. Known formats include chopped (molding compound), unidirectional sheet or tape, continuous strands (straight or braided), or woven cloth. Examples of materials typically used for the reinforcing fibers include carbon, graphite, boron and silicon carbide.
Polymer matrix composite fiber strands are often used in filament winding fabrication processes wherein the continuous fiber strands are wound over a rigid form or mandrel (which may or may not be subsequently removed) to form an assembly which is then cured. In some applications, the fiber composite strands are partially cured prior to the winding process. This preliminary curing is known as "staging." Conventional methods for staging composite fiber strands include heating spools of impregnated fibers in an oven and feeding impregnated fibers through a series of pulleys in a heated chamber. The former method results in materials with uneven physical properties, while the latter method requires a relatively large chamber with commensurate energy costs. Many composite materials require a low-oxygen curing environment; such an environment is expensive to provide with the conventional methods.
These conventional staging methods involve exposing the outer surfaces of the part to an external source of heat. Heat transport to the interior of the part occurs by conduction. This process requires that surface temperatures exceed interior temperatures for some or all of the production cycle. The rate of heating is dependent on this temperature gradient (i.e., the greater the temperature gradient, the faster the part will be fully heated). Since the polymeric matrix will be damaged if exposed to excessive temperature, the heating rate must be restrained (thus lengthening cycle time) so that no portion of the part exceeds the temperature limit.
In addition, some polymers undergo condensation-type chemical reactions during curing which produce volatile reactants. The higher exterior temperatures of conventional heating techniques tend to advance curing at the surface of the part, sometimes forming a hard, impermeable "skin" on the part surface which can trap the volatiles given off during the condensation-type reactions. Trapped volatiles usually result in excessive voids and delaminations, essentially rendering the finished part useless.
Induction heating is one approach which can avoid the above-mentioned disadvantages. Generally, induction heating occurs by exposing a workpiece to an oscillating magnetic field. They magnetic field is typically produced by passing an alternating current through a conducting coil situated near the workpiece. The applied field induces eddy currents in the workpiece, and the eddy currents generate heat by resistive effects. The induced eddy currents generally flow perpendicular to the applied magnetic field and tend to roughly mimic the coil current. Thus, to be susceptible to induction heating, the workpiece must be electrically conductive and be able to define electric paths which approximate the shape of the coil. Isotropic workpieces such as metallic plates easily meet these requirements even with spiral coils, but other workpieces are less susceptible to conventional induction heating. Polymer matrix composites containing electrically conductive fibers could conduct electric currents but the currents are essentially restricted to the fibers because of the high resistivity of the polymeric matrix material. Thus, the electrical conducting paths in such composites are limited.
Accordingly, there is a need for an apparatus and method for heating polymer matrix composite fiber strands by induction heating, thereby rapidly heating the composites without the large temperature gradients necessary with conventional conductive heating.